If you are using the Search bar from the top of the page (found in the Discover page on Mobile), typing in different keywords will first load you the most common results members have looked for using that keyword.
If you hit the Enter key (Send button for mobile), you'll be taken to the Search Results page, where you can find a lot more results than the ones shown to you beforehand. Here, you can filter your results in many ways, depending on what you're looking for.
The All filter is the default one after a Search is made. After that, you can choose between seeing only Topics, Posts, People (Profiles), Videos or Photos. You'll have to play around a bit to get a feel for it, but it's not as complicated as it sounds.